This invention relates to an optical information recording and reproducing apparatus.
Conventionally, an optical information recording and reproducing apparatus, using an optical disk on which tracks are formed spirally or concentrically, used a method of focusing a light beam from a light source such as a laser onto a track by means of an optical pickup device in order to record information on the tracks and to reproduce the recorded information. When using a recording and reproducing apparatus, the focusing of the light beam spot correctly on a desired track was performed by using a focusing control to move the focus of the light beam and a tracking control to match the focused position of the light beam with the track. If the surface of the optical disk was inclined or warped, or if the adjustment of the optical system of optical pickup device deviated, an offset appeared in the tracking control signal causing the tracking to operate unsatisfactorily. This problem is clearly described below using FIG. 2 through FIG. 7.
FIG. 2 is a simplified block diagram of a tracking servo system for tracking control. A photo detector for tracking control 1 detects the deviation of the light beam in the tracking direction. In this example, the circuit is divided into two parts. a subtractor 2 determines the difference of the output of this photo detector 1. An actuator driving circuit 3 comprising a phase compensation circuit and others drives an actuator 4. The actuator 4 shifts an objective lens 5 in the tracking direction. The output of the subtractor 2 is called a tracking control signal TES. On the optical disk, as mentioned above, tracks 7 are formed spirally or concentrically, as shown in FIG. 3, in order to record information on the optical disk 6.
FIGS. 4(a-c) are a schematic representation of the changes in the quantity of light incident upon the photo detector based on the relation between the focused position of light beam and the track position.
A light beam 8 is focused by the objective lens 5 onto the track 7 of optical disk 6 and is reflected and diffracted back to the photo detector of tracking control 1, appearing as the quantity of light 9 received on the photodetector. The hatching in FIGS. 4(a) and 4(c) schematically shows the larger quantity of light. Depending on the focused position of light beam 8 and the position of the track 7, the quantity of light 9 received on the photodetector 1 varies which in turn causes the output of the subtractor 2 (tracking control signal TES) to vary positively or negatively.
The quantity of light received at the photodetector for tracking control 1 changes as shown in FIGS. 4(a-c) depending on the relative positions of the light beam focused position and track. When the output difference of the elements of the photodetector 1 (two elements are shown in this example) is obtained from the subtractor 2, a track control signal TES varying positively or negatively depending on the relative positions is obtained.
A positive change is shown in FIG. 4(a), and a negative change is demonstrated in 4(c), while 4(b) denotes a case of exact tracking of light beam on the track, causing the tracking control signal TES to be zero.
Usually, the tracking control system controls the focused light beam position so that the TES is zero near the middle of a track causing the light beam to be correctly positioned on the track.
When a light beam vertically enters the disk surface without inclination, according to the changes in the relative positions of track 7 and light beam 8, a tracking control signal TES of an approximately sinusoidal waveform and adjusted to 0 V when the focused light beam is in the middle of a track and at the intermediate part of tracks will be obtained as shown in FIG. 5(a). The tracking servo system controls the light beam position so that this tracking control signal TES is zero near the middle of a track, so that the light beam will be focused at the middle of a track. If, however, the light beam is not emitted vertically onto the disk surface due to inclination or warp of the disk, the tracking control signal will not be 0 V when the focused light beam is in the middle of the track as shown in FIG. 5(b) causing an offset of Voff.sub.0 to occur as a result. Accordingly, the light beam is focused at a position off from the middle of the track by the portion of Xoff.sub.0.
If the adjustment deviates in the optical system of optical pickup device, a similar phenomenon occurs. FIG. 6 shows a properly adjusted optical system where the center of the light beam 8 coincides with the middle of the track 7, and the quantity of light received 9 is equally divided on elements j and k of the photodetector 1, so that the tracking control signal TES, the output of the subtractor 2, is zero. As stated above, since the servo system is to control the focused light beam position by making the tracking control signal TES zero, the light beam is correctly controlled to be positioned in the middle of the track in this case. When the adjustment of the optical system deviates, as shown in FIG. 7(a), and the light beam is correctly converged on the middle of the track, an offset of Voff.sub.1 occurs in the tracking control signal TES. In response to this offset, the servo system operates in such a manner to produce a tracking control signal TES of zero. This causes the center of the light beam to be focused on a position off from the middle of the track, Xoff.sub.1. Thus, when the center of the light beam is focused off the middle of the track, the information cannot be recorded or reproduced correctly. Moreover, if there is material that is different optically or physically, in one or more positions than in other parts on the track (for example, a disk having sector number or other information formed in a shape different from others in part of the track on which the track number is formed as pits), an abnormal voltage spike appears in the tracking control signal TES when the light beam comes to the boundary of these two parts or when going out of this boundary, due to the difference in optical or physical properties between these two parts causing unstable tracking control or other inconveniences (see FIG. 12).